[hist] Implement RHistStats

[hahnjo]

Histogram statistics of unbinned values to compute the number of effective entries as well as the arithmetic mean and other statistical quantities per dimension.

[github-actions]

Test Results

    22 files      22 suites   3d 16h 8m 56s ⏱️
 3 684 tests  3 684 ✅ 0 💤 0 ❌
79 150 runs  79 150 ✅ 0 💤 0 ❌

Results for commit cb378c8.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 92.55319% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.81%. Comparing base (f80fd60) to head (9ba5607).
⚠️ Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
hist/histv7/inc/ROOT/RHistStats.hxx	92.55%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19899      +/-   ##
==========================================
+ Coverage   35.80%   35.81%   +0.01%     
==========================================
  Files        3387     3389       +2     
  Lines      515944   516219     +275     
  Branches   242384   242503     +119     
==========================================
+ Hits       184730   184884     +154     
- Misses     218442   218526      +84     
- Partials   112772   112809      +37     

Flag	Coverage Δ
unittests	35.81% <92.55%> (+0.01%)	⬆️

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[hahnjo]

Ping, this is the last PR needed before I can put together the "final" RHist = RHistEngine + RHistStats

[jblomer]

LGTM. I trust you on the math (it helps that the trickier parts are cross-checked).